1. Field of the Invention
This invention relates to an interferometric method for dimensional measurements and defect detection. More particularly, the invention is a variable phase contrast interferometric method permitting dimensional measurements without complex computer calculation and also defect detection.
2. Description of the Related Art
In practically all fields of technology, the processing tolerances observed during the manufacture of parts have been tightened considerably. This trend has been most evident in the manufacture of integrated semiconductor circuits in the submicron range and for the manufacture of magnetic and optical storage disks, since the extreme miniaturization and increased packaging densities necessitate extremely plane and flawless surfaces. To meet these tightened processing tolerances, extremely accurate, high speed, and automatic measuring techniques are required for the control of materials, for the monitoring of production processes, and for final testing.
Past methods used for dimensional measurements include ellipsometric and stylus techniques. Ellipsometric methods use polarized radiation incident on the surface to be measured at a large angle, the ellipticity of which is measured after reflection at the surface measured. The necessary complicated mathematical calculations are so time consuming that it is not possible to examine a great number of measuring points on one sample for manufacturing control for final test during production. Stylus measurement techniques involve the running of a sensitive electromechanical stylus across a surface to be measured. However, the mechanical contact inherently required by this technique makes it impractical for use on the delicate surfaces of miniaturized semiconductor circuits or data recording disks.
A variety of interferometric techniques are also known for making dimensional measurements. In general, these techniques involve the observation of the interference pattern created by light beams reflecting off the surface being observed Some interferometric techniques include the use of light beams of at least two different wavelengths. Examples of multiple wavelength interferometric methods are shown in U.S. Pat. Nos. 4,652,744 and 4,552,457. Such multiple wavelength techniques suffer from the disadvantage that their measurements depend on the amount of light reflected from the observed surface, which is determined in part by the focal plane of each of the incident light beams thereon The flatness of the observed surface thus limits the applicability of such techniques for use in making dimensional measurements, such as step heights.
Amplitude shearing interferometry may also be used for making dimensional measurements. This technique employs two coherent light beams generated from the same source. To measure the step height on the surface of a specimen, for example, the two light beams are made to reflect from the specimen surface on different sides of the step. The step causes a difference in the path length of the two light beams. By observing the shift in the interference pattern of the two light beams caused by the step, the amount of the shift can be geometrically translated into the step height. Amplitude shearing techniques suffer from two disadvantages. First, such measurements can only be made where the step height does not exceed half the wavelength of the light used. This is because the amount of shift in the interference pattern for such step heights prevents the unequivocal location of the interference maxima and minima. Another disadvantage of amplitude shearing interferometry is that the measurement of small step heights is limited by one's ability to resolve the shift in the interference pattern. The resolution of the interference pattern shift is limited to approximately one-tenth of the wavelength of the light used because multiple light waves will actually be detected by the system photo detector.
Phase shearing interferometry is a technique with improved interference resolution compared with that of amplitude shearing interferometry. Phase shearing interferometry has been disclosed in U.S. Pat. No. 4,298,283 and a related publication. Makosch, G., Solf, B., "Surface profiling by electro-optical phase measurements", SPIE Vol. 316 High Resolution Soft X-Ray Optics (1981), pgs. 40-53. The technique uses a polarized light beam which is passed through an electro-optical phase modulator and resolved into two orthogonally polarized beams using beam splitting optics consisting of a Wollaston prism and a focusing lens, the two laser beams are focused on the object surface as colinear beams. The two beams reflect from the object surface on opposite sides of the step height therein. The reflected beams are combined by the Wollaston prism and are brought to interference passing through a polarizer preceding the photodetector. The step height may be calculated mathematically from the phase shift imparted between the two beams. The phase shift is itself calculated from the intensity of the beams measured by the photodetector. The light intensity measured at the photodetector varies sinusoidally with the voltage applied to the phase modulator. By measuring the detected intensity at three different voltages, the phase shift and hence the step height can be calculated without variations caused by reflectivity of the object surface This technique is an improvement over amplitude shearing interferometry in that resolution to approximately 1/300 of the wavelength of light used can be achieved. U.S. Pat. No. 4,358,201 and European Patent Application No. 0226658 also show phase shearing interferometers with some modification, such as the use of a Foucalt prism in place of the Wollaston prism.
Although phase shearing interferometry allows for improved resolution, the technique suffers from the disadvantage that three intensity measurements at different voltages, and the calculations associated therewith, result in slow processing times. The time required for measurements is such that the tool is not suitable for use in the manufacturing environment. The MP-2000 non-contact surface profiler tool, marketed by Photographic Sciences, Corp., uses a technique similar to phase shearing interferometry for measuring surface roughness in the manufacturing environment. The tool is similar to a phase shearing interferometer, except that no electro-optical phase modulator is used. Instead, a differential detector is used after the beam is deflected by a polarized beam splitter. However, because this tool only makes a single intensity measurement, variations in surface flatness and reflectivity at each point of reflection will affect the intensity detected. For this reason, the beams incident upon the object surface are brought extremely close together, thereby making the tool impractical for use in dimensional measurements such as for step height. Information on the MP-2000 tool was acquired from a 1987 product brochure, which lists the following address for contact to receive additional information: Photographic Sciences Corp., 770 Basket Road, P.O. Box 338, Webster, N.Y. 14580-0338.
Other interferometers are known in which the voltage applied to a light beam phase modulator is selected so as to maximize the resolution achievable by the system. For example, U.S. Pat. Nos. 4,286,878 and 4,280,766 disclose fiber optic interferometric gyrometers. These gyrometers sense rotation by measuring the difference in time it takes for light or other electro-magnetic waves to pass in opposite directions through a common path loop whose rotation is to be measured. If the two beams are only slightly out of phase, the light intensity at the detector will not vary significantly for small changes in phase difference between the two beams because the light intensity versus phase difference curve will be at a location of substantially zero slope. To resolve this problem, the references disclose the use of a phase difference imparted to the two light beams reaching the detector, such that the beams are 90 degrees out of phase at a zero rotation rate of the fiber optic loop. This assures that the system operates at a point on the light intensity versus phase difference curve wherein any slight change in phase difference between the beams of the pair results in a substantial change in the intensity of light falling on the detector (i.e. at or near the point of inflection of the curve). However, neither these or any of the aforementioned references, provide for both dimensional measurements and defect detection in the manufacturing environment.